This invention relates to an apparatus for controlling internal pressure overloads in a hydraulic pump. More particularly, the apparatus is preferably adaptable for use in a piston-actuated diaphragm pump, whereby pressure unloading must be utilized to prevent damage to the diaphragm in cases where the pumped fluid pressure fluctuates, either because of a fluid flow shutoff downstream or because of a drop in fluid supplied to the pump. For example, if a hydraulic diaphragm pump is utilized to pump water, paint, or some other fluid through a hose and nozzle configuration for spraying or cleaning, and if the nozzle is triggered off, the hydraulic pressure immediately builds up in the delivery hose and feeds back to the diaphragm pump, wherein it is detected as an accumulation of fluid in the pumping chamber acting in opposition to the diaphragm. Since a diaphragm pump typically has a mechanically reciprocated piston, the compression stroke of the piston will pressurize the hydraulic oil on the pumping side of the diaphragm in opposition to the blocked pumped fluid chamber. Since the fluids are incompressible, the mechanically actuated piston will suddenly encounter a stall force in the absence of any pressure relieving mechanism. Such a stall pressure could damage or destroy the pump by rupturing the diaphragm or breaking the mechanical moving parts.
This problem is typically resolved by providing a pressure unloading means on one side of the diaphragm or the other. This pressure unloading means usually operates to cause a bleeder valve to open whenever a predetermined pressure level is sensed, and the compressed fluid is allowed to escape into an unpressurized area. If the pressure unloading means is placed on the pumped fluid side of the diaphragm, it merely allows pumped fluid to escape from the pump chamber into an accumulator or recirculating passage. If the pressure unloading mechanism is located on the hydraulic oil side of the diaphragm, it generally permits excess hydraulic oil pressures to be relieved back into the hydraulic oil reservoir associated with pumps of this type. Of course, additional valving means must be provided to enable replenishing of the hydraulic oil chamber whenever such a bleedoff has occurred, in order for normal pumping to again resume after the pressure load has been relieved.
In the prior art, hydraulic diaphragm pumps have utilized a great number of schemes for regulating over pressures of fluid acting against a flexible and sensitive diaphragm. For example, U.S. Pat. No. 2,753,805 discloses a hydraulic diaphragm pump having an auxiliary diaphragm connected to a slide valve which is spring-biased to a preset pressure overload valve. As the pressure in the pumped fluid chamber, relative to the hydraulic chamber pressure, drops below a preset limit the slide valve opens to allow excess hydraulic oil from the displacement chamber to be passed back to the hydraulic oil reservoir. This patent utilizes a number of check and slide valves, and may be considered to be a forerunner of the present invention. It is of particular interest because it provides a means for an auxiliary sensing diaphragm for monitoring pressure differentials between the pumped fluid chamber and the hydrualic oil displacement chamber, and it provides valving to allow hydraulic oil to be bypassed back to the reservoir whenever preset pressure differential limits are exceeded.